Final Project - New Orleans Flood Hazard Map

With the disastrous flooding that occurred in New Orleans during Hurricane Katrina, and now with the incredible destruction witnessed in northern Japan with the Sendai tsunami, the world is now, more than ever, aware of the destructive power of water. The city of New Orleans is famous for its mixture of old European flare and American culture, but asides from its cultural appeal, New Orleans is famous for being below sea level, prone to seasonal flooding from the Mississippi and heavy storm surges from Hurricanes in the Gulf of Mexico. With all of these potential threats to the city of New Orleans, both the city’s government and citizens have grown aware of the threat flooding imposes on the city. Thus, the map I created sets out to find the locations where people and businesses are most under threat of flood and flood damage.

The first step in determining how to construct and create a flood hazard map is to determine the different environmental factors that either deter or attract flooding to certain areas. Like the fire hazard map we made in class, the flood map is a sum of all flood causing factors. Once all these factors are given values, a flood map detailing all potential hazard zones is created. But before this flood hazard map is created, a list of flood hazards has to be constructed. And before these flood hazards are determined, it is also important to find good and reliable data sources to use in constructing this map, as well as a heavy amount of data manipulation that needs to occur before the data can be useful in creating a flood hazard map.

The data source used to make the flood hazard map is that of the GIS database at the Louisiana State University, Baton Rouge. The LA ATLAS (Louisiana State GIS database) acts as the central GIS database of the state of Louisiana, and is the most reliable and reputable source of shape files and raster data for the state of Louisiana, as well as for the greater New Orleans metropolitan area.

After determining the location of a reliable source of data, the search for quality and useful data, information important and relevant to creating a flood hazard map of the New Orleans metropolitan area begins. The data I downloaded included shape files on the location of levees, bodies of water, land use, elevation, soil type, and toxic waste dumps.

The first download was a levees shape file, provided by the LA Atlas, made by the USACE or United States Army Corps of Engineers, the federal branch of government in charge of constructing and maintaining the levee system. This is a polyline dataset, as levees are linear in nature. But like many of the other datasets, the coverage of the levee system is statewide, and using the clip tool, the data was clipped to cover only the New Orleans metropolitan area.

The second dataset downloaded was that of hazardous waste sites in the State of Louisiana. Collected by the EPA and mandated by law to be open to the public, locations of all sites where hazardous materials can enter the environment is provided in a shape file. Though toxic waste sites don’t cause floods, flood waters can damage these sites, potentially leaking out dangerous chemicals into the environment creating a more hazardous environment in the surrounding area.

The third file downloaded contained the locations of bodies of water in the State of Louisiana. This polygon map provides a map of every known lake, river, or marsh area in Louisiana, and like the levee map, needed to be clipped to provide coverage of the New Orleans metropolitan area. Along with the hazardous waste sites and levees, buffers were made along the perimeters. For the water features polygon, 1 mile wide buffers were made, indicating the areas flood waters could reach in the event that the Mississippi River or Lake Pontchartrain swells its boundaries due to heavy rains or storm surges. The levees were given thin 100 yard buffers because the buffer around water features already exposes the potential damage from flooding near levees, since levees run around water features. Instead, the purpose of the levee buffers is the potential of levee failure. Though flooding may inundate a home or building with water, many times, the structure of the building remains intact. With levee failure, a torrent of water rushes through the small gap in the levee, destroying anything in its path, such as the recent tsunami did in Northern Japan. Thus anybody or anything within a close proximity to a levee is under more threat of destruction due to the sheer force and power of flood waters near broken levees. The toxic waste site was given a half mile buffer because though floodwaters are dangerous and can carry these toxins, the chance of these toxins being diluted in the flood waters are much greater as you move further away from the original toxic waste site.

After creating these buffers, it was time to download raster data. The next data set was on elevation. I downloaded a 1 arc second DEM from the USGS seamless viewer. The elevation data set is given by far the most weight in the flood hazard calculation map. The reason for such high value being given is because of the fact that New Orleans is shaped like a bowl, and if there is any breach in the levees or even a heavy rain, all of the water is not going to drain to the local lakes or rivers, but will drain to the areas of lowest elevation. Large pumping stations located throughout the city of New Orleans were built to handle the water, but these pumping stations are incapable of handling such large amounts of flood water. Thus elevation soon becomes the most important factor in determining flood hazard zones.

The next data I downloaded was data on soil types. There are four main soil types in the New Orleans metropolitan area, all with different ability in absorbing and retaining moisture. Soils that were historically located under marshes are rated with the highest value, in that many of the soils are already fully saturated with water, unable to absorb any new moisture. On the other hand, alluvial soil located on the natural levees of the Mississippi has a greater ability to absorb and retain moisture as they are newer and above sea level.

Lastly I downloaded a dataset on land use. Land such a swamps and marshes were rated very low since they are already flooded, as well as urban landscapes, since concrete and asphalt both do not absorb water and allow floodwaters to travel faster than if they were flowing across other land types such as agricultural land or forested areas.

With most of the data besides the elevation dataset, I used a valuing system from zero to three or four, because besides elevation, the data are equally as hazardous. But with the elevation data set, I subtracted from the overall sum in areas with higher elevation, because the potential of flooding in areas above sea level only occurs in only the most extreme of circumstances.

The resulting flood hazard map is a calculation of all values given to certain map features using the reclassify method in the spatial analysis toolbar. In measuring the accuracy of this map I compared the results of the flood hazard map to the flooding which enveloped New Orleans after Hurricane Katrina. As with flooding after Katrina, the areas darkest (and thus more prone to flooding) were the areas hardest hit, and where the majority of the damage and destruction occurred.

Though this map includes many elements, improvement to the map can be made. With more research into the impact on flooding different elements have, a more precise flood hazard map can be created. These different elements, such as the locations of water pumps, storm drains, age of the levee system, can all be added to the map. But what this map does do is give a general knowledge of the locations that will be hardest hit by flooding in case of any major storm, giving public officials and citizens the information necessary to react quickly and distribute supplies where necessary, if and when New Orleans is hit by the next ‘Big One’.